I have experimented with molds of the type described in U.S. Pat. No. 3,940,229 for the purpose of making concrete masonry units with a roughened texture on at least one face. In this type of mold, one of the walls of the mold includes an inwardly extending lip on the lower edge of the wall. The specification of the '229 patent describes this lower lip as producing a scraping or tearing action on the adjacent surface of the green concrete masonry unit as it is stripped from the mold, to produce a roughened texture on the finished product. In my observation, the lower lip acts by retaining a portion of the fill material in place against at least a portion of the associated mold wall as the mold is stripped. Thus the lip catches some of the aggregate in the material, and pulls, or rolls, it up the side of the green block as it is stripped from the mold, thus causing the roughened surface.
As I experimented with this mold, the thought occurred to me that I might get an improved roughened face if I positioned an upper lip along the same wall carrying the lower lip. My thought was that an upper lip of the same depth as the lower lip, positioned just at the compacted fill level of the mold cavity, might block fill from "squirting" out between the mold wall and the stripper shoe as the mold was stripped from the block. Of course, the more I thought about this, I realized that, as the mold was stripped, this upper lip would be moving progressively further away from the molded block, so that the effect which I at first envisioned couldn't occur as I envisioned it. Nonetheless, I decided to experiment by positioning an upper lip as described.
When I produced blocks in the mold with the additional upper lip, it appeared to me that a somewhat rougher-textured block was produced than was produced in the mold without the upper lip. To date, I have no definitive explanation for why this occurs. My present theory is that the upper lip somehow interacts with the mold vibration to produce more compaction of the material adjacent the associated wall than is the case when no upper lip is employed, and that this improved compaction at the wall enhances the roughening effect of the lower lip. This is consistent with my observation of the mold cavity immediately following stripping of the mold. In the case of a mold having only a lower lip, I observed that some of the fill material remained adhered to the end wall above the lower lip. This material extended approximately halfway up the wall along its entire length, and was somewhat discontinuous in its coverage. In the case of a mold having both an upper and a lower lip, I observed that more fill material remained adhered to the end wall between the upper and lower lips, that it was a thicker, more compacted layer of material, and that it was more continuous in its coverage. In both cases, when a new pallet is positioned against the bottom of the mold--the pallet typically slaps the bottom of the mold as it moves into position--the material adhering to the end wall is generally knocked loose from the wall.
Not only did the upper lip act to produce a somewhat rougher surface, but it also provided a useful alignment guide for positioning of the stripper shoe, so that it would not interfere with the lower lip as the mold is stripped.
I am also aware of U.S. Pat. Nos. 5,078,940 and 5,217,630, which also describe a mold like that shown in the '229 patent having a lower lip on one wall to produce a rough textured surface on a concrete masonry unit. The '940 and '630 patents describe the use of a screen and a series of projections on the mold wall to hold fill material against the wall as the mold is stripped. I believe that maintenance of such a screen would prove difficult in a typical production environment, and that the use of such a screen and projections would result in a mold that is not self-cleaning, and will require frequent stoppages in production to clear before material becomes unacceptably hard against the wall.
My mold does not have either of these problems.